This invention relates to containers, and more particularly to a plastic container used, for instance, as a can for packing food.
Metal cans (or tin cans) have been extensively use for packing, for instance, food; however, recently the tendency is decreased, because metal cans are cold, poor in appearance, and adversely affect the taste or flavor of the food therein.
Therefore, there has been a strong demand for the provision of cans which are made of other materials and are free from the above-described difficulties. In order to meet this requirement, a multi-layer film comprising an aluminum foil and a plastic layer formed on the alter has been used to form a container; that is, a cup-shaped container for packing contents such as for instance food has been formed by using a flat-plate-shaped multi-layer film (in the form of a sheet).
One of the conventional method of forming a cup-shaped container or container body by using such a flat-plate-shaped multi-layer film is a drawing method. In the drawing method, with the periphery of the multi-layer film clamped on the stand, pressure is applied to the film from above to draw the latter downwardly, to thereby form a container body deep enough to receive the contents. Normally, the operation is carried out by press cold drawing.
The method is disadvantageous in that since the multi-layer film is drawn, the aluminum foil becomes nonuniform in thickness, and is liable to have pin holes or cracks, and accordingly it is not practical to use a thin aluminum foil; that is, an aluminum foil as thick as 60 to 100.mu. must be used with a result that the can manufacturing cost is increased as much. Furthermore, since the aluminum foil is drawn, it is impossible to form a deep container.
In addition, it is difficult to increase the wall thickness of the container, and accordingly the resultant container is liable to be deformed when sue or transported.
Another example of the method of forming a cup-shaped container body by using the flat-plate-shaped multi-layer film is as follows: The film is cut into film pieces having predetermined configurations, and the film pieces are bonded together with an adhesive or the like to form the container body. However, the method is disadvantageous in that, when the container is filled with contents such as food, the edges of the film pieces; i.e. the edges of the metal foil and adhesive layers contact the contents; that is, the method is not acceptable in food hygiene. In addition, the container formed according to the method cannot sufficiently withstand retort sterilization.
In view of the foregoing, the present inventors have proposed the following method of forming a cup-shaped container body by suing a multi-layer base material such as the above-described multi-layer film: In the method, a cup-shaped container base is formed before the container body. If the multi-layer base material is not drawn nor cut to form the cup-shaped container base, then unavoidably it will have unnecessary portions which do not contribute to the formation of the container base. However, in the method, the unnecessary portions of the multi-layer base material are absorbed as wrinkles. That is, in the formation of the container base, the multi-layer base material is not subjected to drawing. And, a resin layer is formed on the outer surface of the container base by injection molding, to form the container body. In this operation, the wrinkles are flattened and smoothed by the resin injecting pressure. Hereinafter, the method will be referred to as "a proposed method", when applicable, for convenience in description.
That is, the specific feature of the container body proposed by the applicant previously resides in that the multi-layer base material comprises an intermediate layer which is a metal foil layer, and resin layers formed on both sides of the intermediate layer which are molten when heated, and it is deformed into a cup-shaped container base with its bottom and its cylindrical wall wrinkled being substantially not drawn, and the resin layer is formed on the outer surface of the container base by injection molding (cf. Japanese Patent Application No. 34649/1985). In one example of the above-described method of forming a container body by using the multi-layer base material, the multi-layer base material which is for instance in the form of a disc is placed between a male and a female metal mold which are vertical grooves, and then the male metal mold is pushed into the female metal mold, to form a container base. The container base thus formed is in the form of a cup with a flange having wrinkles extended vertically. The method is advantageous in the following points:
(1) As was described above, the multi-layer base material is set between the male and female metal molds, and it is slid into the female metal mold by pressing the male metal mold to form a container base, and in this operation, unlike the conventional method, the multi-layer base material is substantially not drawn. Therefore, the method can provide a container body mode of a multi-layer plastic base material having an aluminum foil as thin as less than 40.mu., which cannot be formed by the conventional method.
That is, in the conventional method using drawing, pin holes or cracks are formed in a thin aluminum foil. On the other hand, in the above-described method, being free from such a difficulty, a thin aluminum foil can be used to form the container base; that is, the thickness of the aluminum foil can be minimized.
When an aluminum foil is merely drawn, then it becomes non-uniform in thickness. However, in the method proposed by the applicant, the metal foil (or aluminum foil) of the container abase uniform in thickness. Therefore, the metal foil which is most important in forming the multi-layer base material can be minimized in thickness with its barrier characteristic maintained unchanged, and the manufacturing cost can be accordingly reduced. For the same reason, the container formed according to the method can be completely burnt out, and the therefore the so-called "public hazard of cans" can be eliminated.
(2) Furthermore, in forming a cup-shaped container base by using a flat-plate-shaped multi-layer base material according to the proposed method, unavoidably a part of the multi-layer base material becomes unnecessary. However, in the proposed method, the unnecessary part is absorbed as wrinkles. The container base with the wrinkles is subjected to injection molding. Therefore, it is unnecessary to draw the multi-layer base material. Furthermore, the inner surface of the container base is pushed against the metal mold by the resin injecting pressure, as a result of which the wrinkles are flattened and smoothed. In the proposed method, unlike the conventional method in which the film is cut into film pieces, and the film pieces are connected together with adhesive or the like to form a container body, the cut edge of the container body will not contact the contents therein. Thus, it can be said that the container body is excellent in food hygiene. Furthermore, the injected resin layer, being formed by injection molding, is positively joined to the outer surface of the container base. Thus, the resultant container body is high in mechanical strength. Accordingly, the container bodies are prevented from being deformed or collapsed when transported or stacked; that is, they are highly appreciated in practical used. And, for the same reason, the sealed container can sufficiently withstand the retort sterilization.
(3) The container base formed according to the proposed method has the resin layer, which is molten when heated, on its side which is opposite to the side covered with the injected resin layer. That is, the container body is excellent in heat sealing characteristic. Therefore, a lid formed of a suitable material can be readily welded to the container body, to form a sealed container. The sealed container thus formed can be subjected to retort sterilization, and can be stored for along period being excellent in gas barrier characteristic. Thus, the container can be widely used for packing food, soup, drink, etc.
(4) In addition, both sides of the multi-layer base material used to form the container are the resin layers. Therefore, the container formed according to the proposed method is free from the difficulty accompanying a conventional metal can that it will adversely affect the flavor of the food therein. Furthermore, the external appearance of the container can be improved by using other resins or changing the configuration.
The container body formed according to the proposed method, unlike the container (body) formed by using only resin injected or merely by three-dimensionally deforming the multi-layer base material, is made up of the container base formed by using the multi-layer base material and the resin layer formed by injection molding. Therefore, if the injection molding method is suitably modified, then the injected resin layer may be so formed that it forms an injected resin flange around the outer periphery of the flange of the container base; that is, the injected resin flange and the flange of the container base form a lid sealing region (cf. the specification of the aforementioned Japanese Patent Application No. 34649/1985).
If summarized, the container according to the proposed method is advantageous in that it can be stored for along period and can be subjected to retort sterilization, and if the lid is firmly welded to the container body, the container will not be broken or blasted if dropped or during retort sterilization. However, it is still disadvantageous in that it is rather difficult to peel the lid off the container when it is required to open the container; that is, it is low in so-called "easy peeling characteristics".
This difficulty may be eliminated by weakly welding the lid to the container body. However, this will cause another difficulty that, when the container is dropped or subjected to retort sterilization, the lid may peel off.